Reactive isocyanate functional (polyurethane or urea forming) adhesives are used for bonding a wide variety of substrates together. Such adhesives are utilized in construction, vehicle manufacture, assembly of electronic subassemblies and devices, toys and the like, and have found widespread use because they allow for reasonable processing conditions and exhibit good adhesion to many substrates. For many substrates the processes used to assemble parts using reactive adhesives require cleaning of the substrate surface to remove contaminants, sometimes the system used to clean the surface is referred to as a wipe. In many processes a primer or activator is utilized to facilitate adhesion of the adhesive to the substrate. A primer or activator system typically contains a film forming resin, a solvent and one or more of an adhesion promoter, catalyst, curing agent or crosslinking agent. See for example DE 19924139. Primers require the formation of a film on the substrate surface which bonds to the substrate surface and to the adhesive surface. Many primers are moisture cure primers which require the presence of moisture to form an adequate film. Examples of primers are U.S. Pat. Nos. 5,115,086; 5,238,993; 5,466,727; 5,468,317; and 5,792,811, all incorporated herein by reference. Non-film forming solutions of adhesion promoters, often called activators, are used which may contain groups intended for reaction with the substrate and the adhesive, examples include U.S. Pat. No. 6,053,971, incorporated herein by reference. Examples of adhesion promoters useful in activators include silanes, isocyanate containing compounds, titanates and zirconates such as disclosed in US Patent 2005/0126683 and US 2006/0124225, all incorporated herein by reference. US 2010/0059179 discloses a system, or kit, comprising i) a stable solution or dispersion of a catalyst for the curing of a reactive adhesive system; and in a separate part ii) an uncured reactive adhesive system wherein the catalyst of part i) accelerates the cure of the reactive adhesive system. Also disclosed is a method of bonding a reactive adhesive to a substrate comprising: a) contacting a catalyst for the curing of the reactive adhesive in a volatile solvent with the surface of the substrate to which the adhesive will be bonded; b) allowing the volatile solvent to volatilize away, c) contacting a reactive adhesive with the surface treated in step a) and d) allowing the adhesive to cure.
One-part isocyanate functional adhesives contain one or more isocyanate functional prepolymers and a catalyst for the cure of the adhesive, which cure as a result of exposure to moisture. The cure rate of a primer and a one part adhesive must be matched to allow the adhesive to link up to the primer. For primers which cure or form a film with exposure to moisture, the adhesive and the primer compete for moisture. With the adhesive applied over the primer, it is often difficult for moisture to diffuse to the primer and the primer may not adequately form a film and the bond of the adhesive system to the substrate may not be sufficient for use.
A two-part polyisocyanate based adhesive comprises, in one part, a polyisocyanate or an isocyanate functional prepolymer and in a second part a curing agent and catalyst for the reaction of the curing agent with isocyanate groups. The curing agent is a compound having on average more than one active hydrogen atom, for instance an amine or hydroxyl moiety. Examples of such systems include those disclosed in the commonly owned patent application titled COMPOSITION USEFUL AS AN ADHESIVE FOR INSTALLING VEHICLE WINDOWS filed in the United States on Jun. 3, 2007 having a Ser. No. 11/824,984 and filed in the PCT on Jun. 5, 2007 having a serial number PCT/US07/013; U.S. Pat. No. 6,965,008; EP 1433802 and EP 1578834, all incorporated herein by reference. Two-part adhesives cure much faster than one-part adhesives. The problem with utilizing two-part adhesives with primers is that the adhesives can cure faster than the primer cures and not link up to the primer or substrate surface. To enhance productivity many users desire fast curing two part adhesives which when applied to substrates can experience bubbling due to the formulation. Bubbling can inhibit adhesion and reduce the strength of the bond. In some processes, the substrate is subjected to abrasive processes, such as grinding to shape or contour the ultimate part for use. For certain substrates, such as fiber reinforced plastics surface treatment may be desirable to enhance adhesion. Surface treatments include mechanical surface abrasion, corona treatment, chemical etching, priming and the like. Treatment of substrate surface can enhance adhesion or can make adhesion more challenging. Also, the adhesive which contains isocyanate groups can tie up moisture needed to cure the primer. The time period between primer application and application of the adhesive to the substrate needs to allow for adequate film build, otherwise the system may not completely link up and form good bonds along all of the layers of the adhesive system.
In modern industrial processes, there is a need to move parts and to place loads on adhesive systems as soon as possible. If this is attempted before the system builds adequate strength is built up a failure of the adhesive system resulting in faulty parts or articles. Fiber reinforced plastics provide the benefits of thermoplastic matrices with higher strength introduced as a result of the presence of fibers. Adhering adhesives to fiber reinforced plastics can be challenging due to use of the different materials and the surface morphology.
What is needed is a system for bonding parts comprising fiber reinforced plastic together or to other substrates using reactive adhesives which avoid these problems and which matches the cure rate of the reactive adhesive to the link up speed to the substrate or the primer system, enhances bonding to abraided surfaces and avoids bubbling.